Merge.hs: restored support for mtl-2.1

As commit 3b0f5c5
didn't transit all modules from Control.Monad.Error
but only one of them it's cheaper to rollback to old module.

Github-bug: #22
Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Merge.hs

@@ -3,7 +3,7 @@ module Merge
   , mergeGenericPackageDescription
   ) where
 
-import Control.Monad.Except
+import Control.Monad.Error
 import Control.Exception
 import qualified Data.ByteString.Lazy.Char8 as BL
 import Data.Function (on)